Method and device for recording an information signal on an information layer of a recording medium

ABSTRACT

The invention relates to a method and a device for recording an information signal  10  in a phase change layer of a recording medium. A mark is recorded by a sequence  13  of write pulses and previous marks between the marks to be recorded are erased by a sequence of erase pulses  14 . De bias power level  191, 192  between the erase pulses is made dependent on the recording speed, thus obtaining low jitter of the recorded marks and at the same time preventing thermal cross write during erase.

This application is a continuation of U.S. Ser. No. 09/850,360, filedMay 7, 2001, now U.S. Pat. No. 6,600,709.

The invention relates to a method of recording at a recording speed aninformation signal on an information layer of a recording medium byirradiating the information layer by means of a radiation beam, saidinformation layer having a phase reversibly changeable between a firststate and a second state, which method comprises a write step forapplying the radiation beam, in response to the information signal, to afirst area of the information layer to cause the first area of theinformation layer to assume the first state, thereby forming a mark, andan erase step for applying a pulsed radiation beam to a second area ofthe information layer, before and after the mark, to cause the secondarea of the information layer to assume a state substantially identicalto the second state, the pulsed radiation beam comprising erase pulseshaving an erase power level (P_(e)) and a bias power level (P_(b))between the erase pulses.

The invention also relates to a recording device for recording at arecording speed an information signal on an information layer of arecording medium by irradiating the information layer by means of aradiation beam, said information layer having a phase reversiblychangeable between a first state and a second state, the devicecomprising a radiation source for providing the radiation beam andcontrol means for controlling the power of the radiation beam such thatin response to the information signal the radiation beam is applied to afirst area of the information layer to cause the first area of theinformation layer to assume the first state, thereby forming a mark, andsuch that a pulsed radiation beam comprising erase pulses having anerase power level (P_(e)) and a bias power level (P_(b)) between theerase pulses is applied to a second area of the information layer,before and after the mark, to cause the second area of the informationlayer to assume a state substantially identical to the second state.

A recording method of the kind set forth in the preamble is known fromU.S. Pat. No. 5,291,470. A mark is written by a sequence of write pulsesand the previously written marks between the marks being written areerased by applying a sequence of erase pulses. The write pluses are at awrite power level (P_(w)) and the erase pulse are at an erase powerlevel (P_(e)), the erase power level (P_(e)) being lower than the writepower level (P_(w)). The power level between the write pulses as well asthe erase pulses is a bias power level (P_(b)). In general, this biaspower level (P_(b)) is set equal to the reading power level (P₀).

The above method is known as a “pulsed erase” recording method asopposed to a recording method where the previously written marks betweenthe marks being written are erased by applying a radiation beam having aconstant erase power level. Because of this constant erase power level,the temperature of the information layer rises gradually. Thistemperature rise leads to erasure of not only marks in the actual track,but also of marks in adjacent tracks. This so-called “thermalcross-write” effect is reduced by applying the pulsed erase method.

It is a drawback of the “pulsed erase” recording method that it does notachieve sufficiently low jitter in the read signal obtained from readingmarks recorded by using the “pulsed erase” method, especially when themarks are recorded at a variable recording speed. The jitter is thestandard deviation of the time differences between level transitions ina digitized read signal and the corresponding transitions in a clocksignal, the time difference being normalized by the duration of oneperiod of said clock.

The recording speed may vary, for example, when recording at a ConstantAngular Velocity (CAV). In this case the recording speed, i.e. the speedat which the recording medium moves relative to the spot of theradiation beam on the recording medium, increases when going from thecenter towards the periphery of a disc shaped recording medium. Therecording speed is also known as the Linear Velocity.

It is an object of the invention to provide a method according to theopening paragraph for recording marks with reduced jitter whilepreventing “thermal cross-write” during erase.

This object is achieved by a method of the kind set forth which ischaracterized in that the bias power level (P_(b)) depends on therecording speed.

When the bias power level (P_(b)) is set at a fixed level no thermalcross write occurs while a minimal jitter of the recorded marks isobtained at a specific recording speed only. However, when the biaspower level (P_(b)) is set in dependence on the recording speed, minimaljitter of the recorded marks may be obtained for each recording speedwhile still preventing the occurrence of thermal cross write to occur.

A more optimum setting can be obtained by introducing additional degreeof freedom in the settings for the sequence of erase pulses (i.e., avariable bias power level instead of a fixed one), resulting in areduced jitter of the recorded marks.

A version of the method according to the invention is characterized inthat the bias power level (P_(b)) increases in a range between zero andthe erase power level (P_(e)) as the recording speed (V) increases.

During recording at a low recording speed the spot of the radiation beamon the recording medium will remain comparatively long in one specificarea of the recording medium compared to recording taking place at ahigh recording speed. Therefore, during recording at a low recordingspeed the area on the recording medium will experience a significanttemperature rise resulting in a significant thermal cross-write. Thistemperature rise, and hence the thermal cross write, is reduced when alow bias power level (P_(b)) is used. However, during recording at ahigh recording speed the spot of the radiation beam on the recordingmedium will remain in one specific area of the recording medium for acomparatively short time only. Therefore, a higher bias power level(P_(b)), even up to the erase power level (P_(e)), may be used withoutintroducing thermal cross write. Application of a high bias power level(P_(b)) results in low jitter of the recorded marks.

A preferred version of the method according to the invention ischaracterized in that the bias power level (P_(b)) increases in a rangebetween zero and the erase power level (P_(e)) as the recording speed(V) increases when the recording speed is below a chosen recordingspeed, and that the bias power level (P_(b)) is substantially identicalto the erase power level (P_(e)) when the recording speed exceeds thechosen recording speed.

Beyond a certain recording speed no thermal cross write occurs, becausethe spot of the radiation beam on the recording medium remains only verybriefly in one specific area of the recording medium. Beyond thisrecording speed the bias power level (P_(e)) is set substantially equalto the erase power level (P_(e)), resulting in a radiation beam having aconstant erase power level. Because of this constant erase power levela. Jitter of the recorded marks is minimized.

A version of the method according to the invention is characterized inthat the duty cycle of the erase pulses depends on the recording speed.

In addition to changing the bias power level (P_(b)), the amount ofenergy transferred from the radiation beam to the recording medium, andhence the local temperature rise of the recording medium, can also beinfluenced by changing the duty cycle of the erase pulses. Both jitterof the recorded marks and thermal cross can both be minimized by settingthe duty cycle of the erase pulses in dependency of the recording speed.

A version of the method according to the invention is characterized inthat the duty cycle increases in a range between nearly zero and unityas the recording speed (V) increases.

During recording at a low recording speed, the spot of the radiationbeam on the recording medium will remain comparatively long in onespecific area of the recording medium compared to when recording takesplace at a high recording speed. Therefore, during recording at a lowrecording speed the area on the recording medium will experience asignificant temperature rise resulting in a significant thermalcross-write. This temperature rise, and hence the thermal cross write,is reduced when a smaller cycle of the erase pulses is used. However,during recording at a high recording speed, the spot of the radiationbeam on the recording medium will remain in one specific area of therecording medium for a comparatively short time. Therefore, a higherduty cycle of the erase pulses, even up to the unity, may be usedwithout giving rise to thermal cross write.

It is to be noted that the above methods according to the inventionoffer especially good results when used in combination with a recordingmedium having an information layer where the recrystallization processof the materials is determined by growth from the edges of amorphousmarks. These so-called Fast Growth Materials (FGM) allow for highrecording speeds but are sensitive to thermal cross write.

It is a further object of the invention to provide a recording device ofthe kind described in the opening paragraph which is arranged to carryout the method according to the invention.

This object is achieved by a recording device of the kind set forth inthe preamble which is characterized in that the control means set thebias power level (P_(b)) in dependence on the recording speed (V).

A preferred embodiment of the recording device according to theinvention is characterized in that the control means set the bias powerlevel (P_(b)) so that it increases in a range between zero and the erasepower level (P_(e)) as the recording speed (V) increases when therecording speed is below a chosen recording speed and that the controlmeans set the bias power level (P_(b)) so as to be substantiallyidentical to the erase power level (P_(e)) when the recording speed isabove the chosen recording speed.

The objects, features and advantages of the invention will be apparentfrom the following, more specific description of embodiments of theinvention as illustrated in the accompanying drawings; therein:

FIGS. 1A through 1C show diagrams illustrating the time-dependency ofthe information signal and of the control signal controlling the powerlevels of the radiation beam,

FIG. 2 shows a graph representing a setting of the bias power level as afunction of the recording speed, and

FIG. 3 shows a diagram of a recording device according to the invention.

FIG. 1 shows diagrams with two types of signals, that is, an informationsignal 10 and control signals 21, 22 and 23 controlling the power of theradiation beam. The diagrams are shown at an identical scale. Thevertical dashed lines in these diagrams indicate transitions in a clocksignal of a data clock belonging to the information signal. The periodof the data clock, also called a channel bit period, is indicated by T₁,T₂, and T₃, respectively. T₁ is the length of one period of the dataclock when recording the information signal 10 at a first recordingspeed. T₂ is the length of one period of the data clock when recordingthe information signal 10 at a higher recording speed amounting toapproximately 1.4 times the first recording speed (T₂=T₁/1.4) and T₃ isthe length of one period of the data clock when recording theinformation signal 10 at an even higher recording speed of approximately1.75 times the first recording speed (T₃=T₁/1.75).

When recording the information signal 10, a ‘high’ period 11 is recordedas a mark and a ‘low’ period 12 is recorded as an unwritten area (aspace) between the marks. A mark is recorded on an information layer ofa recording medium by irradiating the information layer by means of asequence of write pulses 13 having a write power level (P_(w)). A spaceis formed by irradiating the information layer by means of a sequence oferase pulses 14 having a erase power level (P_(e)) or by irradiating theinformation layer by means of a radiation beam having a constant erasepower level 15 (P_(e)) thus erasing any previously recorded marks. Thismechanism of erasing a previously recorded information signal while atthe same time recording a new information signal is known as DirectOverwrite.

The shapes of both the write pulse sequence 13 and the erase pulsesequence 14 are shown as examples. Other shapes, for example comprisingan alternative number of pulses, may also be used.

FIG. 1a shows the control signals 21 controlling the power of theradiation beam when recording the information signal 10 at acomparatively slow first recording speed. The bias power level (P_(b))191 is set at a power level P₁ such that no thermal cross write effectsoccur when recording at this recording speed.

FIG. 1b shows the control signals 22 controlling the power of theradiation beam when recording the information signal 10 at a higherrecording speed amounting to approximately 1.4 times the first recordingspeed. The bias power level (P_(b)) 192 is set at a power level P₂. Atthis power level P₂ no thermal cross write effects occur when recordingat this higher speed. Moreover, jitter of the recorded marks is reducedbecause P₂ is higher than P₁.

FIG. 1c shows the control signals 23 controlling the power of theradiation beam when recording the information signal 10 at an evenhigher recording speed amounting to approximately 1.75 times the firstrecording speed. At this recording speed the bias power level (P_(b)) isset equal to the erase power level P_(e). Because of the high recordingspeed the spot of the radiation beam on the recording medium will remainonly comparatively briefly in one specific area of the recording medium.Because of this, no thermal cross write effect will occur, not even whena constant erase power level P_(e) is applied. Applying a constant erasepower level P_(e) will result in a minimized jitter of the recordedmarks.

FIG. 2 shows an example of a setting of the bias power level P_(b) as afunction of the recording speed V. The horizontal axis 26 of the graphrepresent the value of the bias power level P_(b) scaled to the erasepower level P_(e) (i.e., P_(B)=P_(b)/P_(e)) and the horizontal axis 27of the graph represents a normalized recording speed V′. Below a chosenrecording speed 29 the bias power level P_(b) increases as the recordingspeed increases. Above the chosen recording speed 29, the bias powerlevel P_(b) is set equal to the erase power level P_(e) for allremaining recording speeds.

FIG. 3 shows a recording device according to the invention for recordingan information signal 10 on a disc-shaped recording medium 30 comprisingan information layer 301. The recording medium 30 is rotated around itscenter by a motor 34. A radiation beam 32 is generated by a radiationsource 31 and focused onto the information layer 301 by a lens 33.

The information signal 10 is applied to control means 60. A currentsource 61 within the control means 60 has four outputs, A, B, C and D.Output A provides a current which, when fed to the radiation source 31through a control signal S_(C), will result in the radiation beam 32having an erase power level P_(e). Likewise, outputs B, C and D providecurrents resulting in the bias power level P_(b), the write power levelP_(w), and the power level in between the write pulses, respectively.The current of each output A, B, C and D can be selected by aswitch-unit 62. The switch-unit 62 is operated by a pattern generator 63controlled by the information signal 10. The pattern generator 63transforms the information signal into a control signal S_(C) comprisingsequences of write pulses and sequences of erase pulses in conformitywith the desired pattern

The control means 60 also comprise setting means 65 for setting thecurrent of output B of the current source 61 in dependence on therecording speed. Setting the current of output B enables the bias powerlevel P_(b) to be set in dependence on the recording speed. Duringrotation of the recording medium 30 at a Constant Angular Velocity(CAV), the recording speed V depends on the radial position of thefocused radiation beam 32 on the recording medium 30. At the peripheryof the recording medium, the recording speed is at its highest and it isat its lowest at the center of the recording medium. The radial positionof the radiation source 31 is controlled by positioning means 35.Information about the radial position of the radiation source 31 istransferred from position means 35 to setting means 65. The settingmeans 65 determine the actual settings for the current of output B ofthe current source 61 on the basis of this information.

FIG. 3 shows an embodiment of a recording device according to theinvention. However, it will be apparent to those skilled in the art thatembodiments of a recording device according to the invention using morepower levels can be realized by adding additional outputs to the currentsource 61 and by extending the switch-unit 62. Moreover, it will beapparent that a single output B of the current source 61 providing avarying current can, as an alternative, be replaced by several outputseach providing a different current while the setting means 65 select theappropriate output for setting the bias power level P_(b) at any time.

What is claimed is:
 1. A method of producing a recording mediumcontaining an information signal, comprising: applying a radiation beam,in response to an information signal, to a first area of an informationlayer of a recording medium to cause the first area of the informationlayer to assume a first state thereby forming a mark, and applying theradiation beam to a second area of the information layer, before andafter the mark, while pulsing the beam to cause the second area of theinformation layer to assume a second state that is different than thefirst state, the pulses including erase pulses having a erase powerlevel (P_(e)) and a bias power level (P_(b)) between the erase pulses,the bias power level (P_(b)) depending on a variable recording speed (V)of the radiation beam along the recording medium.
 2. The method of claim1, in which: the information layer includes a material having a phasethat is reversibly changeable between the first state and the secondstate; the first state is an amorphous state; and the second state is acrystalline state.
 3. The method of claim 1, in which the bias powerlevel (P_(b)) increases in a range between zero and the erase powerlevel (P_(e)) as the recording speed (V) increases.
 4. The method ofclaim 1, in which: the bias power level (P_(b)) increases in a rangebetween zero and the erase power level (P_(e)) as the recording speed(V) increases when the recording speed is below a chosen recordingspeed, and the bias power level (P_(b)) is substantially identical tothe erase power level (P_(e)) when the recording speed exceeds thechosen recording speed (29).
 5. The method of claim 1, wherein: theerase pulses have a duty cycle of T_(e)/T_(b), where T_(e) is theduration of an erase pulse and T_(b) is the time between two successiveerase pulses, and the duty cycle depends on the recording speed (V). 6.The method of claim 5, in which the duty cycle increases in a rangebetween nearly zero and unity as the recording speed (V) increases.
 7. Arecording medium containing an information signal, produced by themethod of claim
 1. 8. A recording device comprising: a radiation sourcefor applying a radiation beam to an information layer of a recordingmedium; means for moving the radiation beam along the information layer;and control means to control the power of the radiation beam: forcausing a first area of the information layer to assume a first state toform a mark in response to an information signal; and for pulsing theradiation beam including erase pulses having an erase power level(P_(e)) and a bias power level (P_(b)) between the erase pulses to asecond area of the information layer, before and after the mark, tocause the second area of the information layer to assume a second statethat is different than the first state, the bias power level (P_(b))depending on a variable recording speed (V) of the radiation beam alongthe information layer.
 9. The recording device of claim 8, in which theinformation layer includes a material having a phase that is reversiblychangeable between the first state and the second state; the first stateis an amorphous state; and the second state is a crystalline state. 10.The recording device of claim 8, in which the control means set the biaspower level (P_(b)) so that it increases in a range between zero and theerase power level (P_(e)) as the recording speed (V) increases.
 11. Therecording device of claim 8, in which: the bias power level (P_(b))increases in a range between zero and the erase power level (P_(e)) asthe recording speed (V) increases when the recording speed is below achosen recording speed; and the bias power level (P_(b)) issubstantially identical to the erase power level (P_(e)) when therecording speed is above the chosen recording speed.
 12. The recordingdevice of claim 8, wherein: the erase pulses have a duty cycle ofT_(e)/T_(b), where T_(e) is the duration of an erase pulse and T_(b) isthe time between two successive erase pulses, and the duty cycle dependson the recording speed (V).
 13. The recording device of claim 12, inwhich the duty cycle increases in a range between nearly zero and unityas the recording speed (V) increases.
 14. A radiation beam controllercomprising: means to control a radiation beam in response to aninformation signal for causing a first area of the information layer toassume a first state to form a mark; and means for pulsing the radiationbeam in a second area between marks, the pulses having an erase powerlevel (P_(e)) and a bias power level (P_(b)) between erase pulses tocause the second area of the information layer to assume a second statethat is different than the first state, the bias power level (P_(b))depending on a variable recording speed (V) of the radiation beam alongthe information layer.